The invention concerns a device for weighting of streamer cables.
During seismic surveys at sea a seismic vessel tows behind it an array of seismic energy sources which transmit seismic signals, and recording equipment for these signals in the form of hydrophones which are arranged in a so-called streamer or streamer cable. This streamer cable, which contains a row of hydrophones, should be kept at a determined depth in the water and should hold a course which is as rectilinear as possible, i.e. so that all the hydrophones are at more or less the same depth. For this purpose the cable must be balanced, in order to achieve the required balance between buoyancy and weight and the cable's depth position is determined by means of guiding wing devices called "birds", which are attached to the cable at regular intervals.
To date, in order to balance streamers weights have been fitted to the outside, and this weighting has consisted of lead pieces or lead plates which are taped to the cable manually. The positioning of these lead pieces has on the whole been based on a rough estimate and, in order to obtain the best possible adjustment of the cable, it has often been necessary to go out in a boat in order to tape on new weights or to move weights.
This method of balancing a streamer has a number of disadvantages. Firstly, the work of taping on the lead pieces is extremely time-consuming, while at the same time the tape dissolves or is destroyed by exposure to the water, causing weights to fall off, resulting in maintenance work and continual renewal of the tape. These materials are expensive, particularly tape of the required quality for use in seal water. Moreover, there is also a regular need to remove the weight, e.g. during transfer from one area to another. This involves a great deal of time-consuming and laborious effort for unwinding tape. It has to be unwound since cutting it off would be too risky as the flexible, soft streamer is easily damaged. The most serious disadvantage, however, is that loose tape or deformed tape generates severe noise and this noise has an extremely unfavourable effect on the recording equipment which is fitted inside the cable. This technical disadvantage combined with the major costs involved in constant renewal of the adhesion of the lead pieces has produced an obvious need for improvement in this stage of the process. In this connection it can also be mentioned that it can be a complicated process to tape on a piece of lead from a small boat in the water.